The present invention relates to a ventilation duct construction, comprising a ventilation duct which is provided with insulation and through which air is allowed to flow, as well as a method of providing a ventilation duct. An increased fire-retardant capability of the ventilation duct is achieved by means of the invention.
Fire-retardant capability is here defined as the capability to resist such a temperature rise of the duct air as causes a temperature rise on the outside of the construction.
To prevent fire from spreading in ventilation ducts, it is common to provide the ventilation ducts with some kind of insulating material, such as rock wool, which is resistant to high temperatures and thus prevents fire from spreading. Conventionally, insulation is arranged on the outside of the wall of the duct and is in most cases provided with an outer metal cover.
The ventilation ducts serve to convey air, but noise also propagates easily. By insulating, at least in some sections of the ducts, the inside of the ducts with sound-absorbing material, such as rock wool or glass wool, and lately also materials which are less fire-retardant, e.g. polyester insulation, it is possible to absorb noise efficiently. Hereinafter, sound-absorbing sections of ventilation ducts will also be referred to as sound absorbers.
In case of fire, it is very important to keep down the temperature of the outside of insulated ventilation ducts or sound absorbers as long as possible to increase the fire safety of the surroundings and close objects.
Since sound absorbers often are enlarged in cross-section compared with other ventilation duct sections and contain insulation with a limited fire-retardant capability, the problems of bulkiness and of obtaining a sufficient degree of fire-retardancy may be accentuated.
Therefore, one object of the present invention is to provide an insulated ventilation duct construction, in which the time during which the outside temperature can be kept down in case of fire is prolonged, while the construction remains compact.
It is also an object to provide a simple method which increases the fire-retardancy of an insulated ventilation duct or duct sound absorber.
According to the present invention, these and other objects which will become apparent in the following are achieved by means of a ventilation duct construction, a sound absorber construction and a method, which have the features stated in the appended claims.
The invention is thus based on the understanding that the fire-retardant capability can be increased by arranging, at least partially, a supplemental thin boundary layer outwardly of the insulated duct or the sound absorber, which boundary layer, on the one hand, provides a supplemental cover configuration having what might be considered as a heat-exchange function and, on the other, provides an additional air insulation effect.
According to one aspect of the invention, a protective or shielding sheet made of metal is thus arranged at least partially around and close to the duct and its associated insulation, not in direct contact with the insulated duct, but instead at a short distance from the same so that an air gap is formed therebetween.
Surprisingly, it has been found that this simple measure imparts a much improved fire-retardant capability to the construction. Preferably, the thermal bridge between the shielding sheet and the duct and its associated insulation is made very small, which can easily be achieved with the aid of suitably designed spacer means. Thus, the time of heating the shielding sheet is prolonged and in particular the time of heating its outside, i.e. the outside of the entire construction.
One advantage of said construction is that the shielding sheet, at least initially, has a cooling effect on the hot air in the gap which has been heated by fire. The thicker the selected shielding sheet, the better the cooling capability. The selection of the width of the air gap as well as the thickness of the shielding sheet must be adjusted to the demands on weight and space. It has been found that a satisfactory effect is obtained even when using a very thin shielding sheet and a very small air gap. The shielding sheet is made of metal, such as galvanised steel sheet or stainless steel sheet. A typical shielding sheet of galvanised steel shows a satisfactory effect even with a thickness of less than 10 mm. The shielding sheet preferably has a thickness of less than a few millimeters, more preferably about 1 mm. The size of the air gap, i.e. the distance between the shielding sheet and the insulated duct, is typically less than 50 mm, preferably less than 20 mm, more preferably less than a few millimeters, such as about 1 mm.
The shielding sheet also has another effect, namely that of distributing the heat over the shielding sheet in case it is locally exposed to a considerable temperature rise. This function is particularly pronounced if the shielding sheet is made of a material having good thermal conductivity.
According to one embodiment, the shielding sheet can be provided with through-holes to improve the circulation of air and heat exchange. This is because cold ambient air is to be drawn into the gap through openings at the ends of the shielding sheet, when the hot air leaks out of the gap through the holes of the shielding sheet. As a result, a circulation of air is provided which contributes to the cooling of the air gap and the shielding sheet. The number, shape and size of the holes can be selected depending on the desired qualities of the shielding sheet. However, the size and the number of the holes have to be chosen, since too big or too many holes could counteract the purpose of the shielding sheet. It has, however, been found that good effects are achieved also when the total area of the holes equals half the area of the shielding sheet.
Depending on the circumstances and the security aspects that must be taken into consideration, the position of the shielding sheet may vary. It is possible to surround an entire ventilation duct of about one hundred meters with a shielding sheet in accordance with the invention, preferably in separate sections so as to provide openings for the intake of air. But it is also possible to provide a ventilation duct with shielding sheet only locally along one or more specially selected sections, such as a sound absorber section. Irrespective of the above-mentioned alternatives, it is not necessary to arrange shielding sheet along the entire circumference of the ventilation duct. It is quite possible that extra protection is needed only along a part of the circumference of the duct.
If the ventilation duct is being passed through a through hole in a wall, a shielding sheet according to the invention may be provided at the location of such a section of the ventilation duct.
Thus, according to another aspect of the invention a protecting or shielding sheet is arranged at least partially around and close to a section of the duct near a wall having a through hole, through which the duct is passed. The shielding sheet comprises a first portion extending essentially in parallel with the ventilation duct, and a second portion extending essentially in parallel with the penetrated wall and being located at an end of said first portion nearest to the penetrated wall. The shielding sheet is thus arranged at such a distance that an air gap is formed between the shielding sheet and both the duct and the penetrated wall.
This second aspect of the invention is particularly advantageous in case of fire on one side of the wall through which the ventilation duct is penetrated. Such a fire may increase the temperature in the ventilation duct on said one side, the rising temperature effect propagating through the duct and thus reaching the other side of the wall. A shielding sheet on the other side will provide the inventive fire-retardant effect.
The first portion of the shielding sheet has primarily a cooling effect on the air in the gap between the ventilation duct and the first portion, while the second portion has primarily a cooling effect on the air in the gap between the penetrated wall and the second portion. Thus, the second portion helps to keep the penetrated wall at a lower temperature than would have been the case without the shielding sheet according to the invention. A relatively long second portion means more cooling capacity.
In one embodiment the first and the second portion of the shielding sheet may be integrated. They may be constructed from one and the same blank or from two different blanks brought together. Alternatively, instead of providing a shielding sheet with an integrated first and second portion, two shielding sheet may be arranged in a spaced apart relationship, i.e. in no contact with each other. One sheet will be in parallel with the ventilation duct and the other sheet will be in parallel with the penetrated wall. This achieves a practical opening between the two sheets for intake and/or outlet of air.
Suitably, insulation is provided in connection with said through hole in the penetrated wall. The insulation may for instance be packed between the outside of the duct wall and the wall defining the through hole. The insulation seals the through hole around the ventilation duct. Any, suitable insulating material may be applied. An example is mineral wool packed in the inner, middle part of the hole and the respective hole opening around the duct insulation is sealed with filling material, such as plaster or the like.
The cross-sectional shape of the ventilation duct or the sound absorber may vary. Typical shapes are rectangular and circular, but other shapes are also feasible, since the invention is not limited to any particular shape of the duct. The shape of the shielding sheet is suitably selected according to the shape of the ventilation duct or the sound absorber, and preferably in such manner that the shapes conform with each other. A shielding sheet according to the second aspect of the invention, i.e. applied at a wall penetration, will besides having a shape (said first portion) that confirms with the ventilation duct, also have a generally flat shape (said second portion) near the wall. If e.g. the ventilation duct is circular, said flat shape may be a circular ring or disc with a hole.
The manner in which the duct is insulated does not limit the scope of the invention. It is, for instance, possible to arrange, on the outside of the wall of the duct, a standard insulation, the outside of which is a metal cover adjacent to which the shielding sheet is arranged with the air gap. A ventilation duct section with sound-absorbing insulation is also possible, the shielding sheet being arranged so that an air gap forms between the same and the outer cover of the sound absorber. Other types of insulation are, of course, also feasible, such as thermal insulation.
As already mentioned, the shielding sheet can be kept at a suitable distance from the ventilation duct with the aid of spacer means so that said air gap forms. As examples of spacer means, mention can be made of pins, screws, rivets and distance plates which are arranged between and fixed to the shielding sheet and/or the wall of the ventilation duct or the insulation placed thereon, with or without a cover. In case of said arrangement in connection with a penetrated wall, spacer means may be arranged between the penetrated wall and the shielding sheet (or second portion of the shielding sheet) parallel to that wall.
The design of the spacer means should allow the thermal bridge to be minimised. Thus the spacer means should be small and limited to a small number. It is also possible to form the air gap without any connection at all, and thus without a thermal bridge, between the shielding sheet and the ventilation duct. One possibility is to equip the construction with magnets so that the shielding sheet is kept xe2x80x9cfloatingxe2x80x9d about the ventilation duct. If there is enough space, another possibility of completely avoiding thermal bridges is to provide the shielding sheet with an outer suspension which is connected to external holding elements.
Even if the shielding sheet has been discussed so far in connection with both ordinary insulated ventilation ducts and duct sound absorbers (ventilation duct sections with sound-proofing), the function of the shielding sheet is the same, irrespective of the type of duct to which it is applied. The use of a shielding sheet according to the invention does not have to be limited to these variants but could also, for instance, be used in connection with heat or condensation insulated ducts.
Besides the discussed ventilation duct construction, the present invention, as already mentioned, relates to a method which increases the fire-retardant capability of an insulated ventilation duct. According to the method, this is achieved by placing a shielding sheet made of metal externally at least partially around the insulated duct at such a distance from the same that an air gap forms between the shielding sheet and the insulated duct. The invention is thus also applicable to existing installations, both to a conventional existing sound absorber and to other ventilation duct sections.
The invention can also facilitate the mounting of a fire-protected sound absorber in a section of a ventilation duct. It is thus possible, for instance, to remove the fire insulation in the desired section, perforate the duct wall to xe2x80x9clet out the soundxe2x80x9d, arrange sound insulation with an outer cover which forms the duct wall on the perforated duct wall and finally arrange the shielding sheet according to the invention around the outer cover so that an air gap forms between the shielding sheet and the outer cover.
One common way of arranging a sound absorber in an existing ventilation duct is to cut off the duct and insert by splicing the sound absorber. The one skilled in the art will realise that a sound absorber equipped with a shielding sheet according to the present invention can also be arranged in a ventilation duct in the same manner.
The invention makes it possible to lower the requirements placed on the absorbing material of a sound absorber as concerns fire-retardancy for the benefit of improved absorbency, resulting in the possibility of reducing the dimensions in the transverse direction, while maintaining the same total fire-retardancy level.